The present invention relates to analysis-by-synthesis speech coding.
The applicant company has particularly described such speech coders, which it has developed, in its European patent applications 0 195 487, 0 347 307 and 0 469 997.
In an analysis-by-synthesis speech coder, linear prediction of the speech signal is performed in order to obtain the coefficients of a short-term synthesis filter modelling the transfer function of the vocal tract. These coefficients are passed to the decoder, as well as parameters characterising an excitation to be applied to the short-term synthesis filter. In the majority of present-day coders, the longer-term correlations of the speech signal are also sought in order to characterise a long-term synthesis filter taking account of the pitch of the speech. When the signal is voiced, the excitation in fact includes a predictable component which can be represented by the past excitation, delayed by TP samples of the speech signal and subjected to a gain g.sub.p. The long-term synthesis filter, also reconstituted at the decoder, then has a transfer function of the form 1/B(z) with B(z)=1-g.sub.p.z.sup.-TP. The remaining, unpredictable part of the excitation is called stochastic excitation. In the coders known as CELP ("Code Excited Linear Prediction") coders, the stochastic excitation consists of a vector looked up in a predetermined dictionary. In the coders known as MPLPC ("Multi-Pulse Linear Prediction Coding") coders, the stochastic excitation includes a certain number of pulses the positions of which are sought by the coder. In general, CELP coders are preferred for low data transmission rates, but they are more complex to implement than MPLPC coders.
In order to determine the long-term prediction delay, a closed-loop analysis, an open-loop analysis or a combination of the two is used. The open-loop analysis is not demanding in terms of amount of calculation, but its accuracy is limited. Conversely, the closed-loop analysis requires much calculation, but it is more reliable as it contributes directly to minimising the perceptually weighted difference between the speech signal and the synthetic signal. In certain cases, an open-loop analysis is carried out first of all in order to limit the interval within which the closed-loop analyser will search for the prediction delay. This search interval must nevertheless remain relatively wide, since account has to be taken of the fact that that the delay may vary rapidly.
The invention aims particularly to find a good compromise between the quality of the modelling of the long-term part of the excitation and the complexity of the search for the corresponding delay in a speech coder.